Date: Tue, 24 Feb 2015 11:25:45 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Pavel Machek <pavel@ucw.cz>
Cc: Jiri Kosina <jkosina@suse.cz>, Vojtech Pavlik <vojtech@suse.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@redhat.com>, Seth Jennings <sjenning@redhat.com>,
        linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Arjan van de Ven <arjan@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Borislav Petkov <bp@alien8.de>
Subject: Re: live kernel upgrades (was: live kernel patching design)
Message-ID: <20150224102545.GD19976@gmail.com>
References: <20150220214613.GA21598@suse.com>
 <20150221181852.GA8406@gmail.com>
 <alpine.LNX.2.00.1502211952120.2357@pobox.suse.cz>
 <20150221191607.GA9534@gmail.com>
 <alpine.LNX.2.00.1502212025450.2357@pobox.suse.cz>
 <20150221194840.GA10126@gmail.com>
 <alpine.LNX.2.00.1502212058080.2357@pobox.suse.cz>
 <20150222084601.GA23491@gmail.com>
 <20150222094639.GA23684@gmail.com>
 <20150223113908.GA3206@Nokia-N900>
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* Pavel Machek <pavel@ucw.cz> wrote:

> > More importantly, both kGraft and kpatch are pretty limited 
> > in what kinds of updates they allow, and neither kGraft nor 
> > kpatch has any clear path towards applying more complex 
> > fixes to kernel images that I can see: kGraft can only 
> > apply the simplest of fixes where both versions of a 
> > function are interchangeable, and kpatch is only marginally 
> > better at that - and that's pretty fundamental to both 
> > projects!
> > 
> > I think all of these problems could be resolved by shooting 
> > for the moon instead:
> > 
> >   - work towards allowing arbitrary live kernel upgrades!
> > 
> > not just 'live kernel patches'.
> 
> Note that live kernel upgrade would have interesting 
> implications outside kernel:
> 
> 1) glibc does "what kernel version is this?" caches 
> result and alters behaviour accordingly.

That should be OK, as a new kernel will be ABI compatible 
with an old kernel.

A later optimization could update the glibc cache on an 
upgrade, fortunately both projects are open source.

> 2) apps will do recently_introduced_syscall(), get error 
> and not attempt it again.

That should be fine too.

Thanks,

	Ingo
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